Japanese Patent No. 2,990,646 (JP-A 8-199070) discloses a composition comprising a straight-chain fluoropolyether compound having at least two alkenyl groups in a molecule and a perfluoropolyether structure in the backbone, an organosilicon compound having at least two H—SiOSiO structures in a molecule, and a hydrosilylation catalyst, which cures into parts having a good profile of heat resistance, chemical resistance, solvent resistance, water repellency, oil repellency, weather resistance and the like.
This fluoropolyether rubber composition can be readily formed into most shapes. In the case of complex shape parts or thin-wall parts like diaphragms, however, it is difficult to remove the molded composition from the mold because of tackiness. Since the difficulty of mold release leads to low production yields of molding step and an increased duration of molding cycle, there is a need for further improvement in processability.
If customarily used mold release agents of the internal addition type for rubber materials based on silicone oil and surfactant (known as internal mold release agents) are incorporated, they invite a rise of viscosity and a loss of fluidity, leading to molding difficulties. It was then proposed to improve mold release by adding a compound containing at least one fluorine atom and at least one hydroxyl group in a molecule and having a melting point of at least 50° C. in powder form as typified by bisphenol AF. This additive is effective for improving mold release, but regrettably exacerbates the compression set which is one of important physical properties of rubber.